Robert Deutsch wrote about the SL-1 Signature Mk.III in March 1998 (Vol.21 No.3):

Since it came on the market in 1985, the Convergent Audio Technology SL-1 has undergone six model changes, making the current Signature Mk.III the seventh official version. (There were also several revisions between model changes.) The basic circuitry has remained much the same throughout, with minor evolutionary changes and improvements in execution. The Mk.II update, which I reported on in December 1996 (Vol.19 No.12, p.144), involved beefing up the power supply, changes in circuit-board construction, and improving the mechanical damping.

Now, for the first time, there's a change in the tube complement: the 12AU7 that had previously served as the line-stage input tube (one per channel) has been replaced by a 6922. The 12AU7 and the 6922 use the same socket and same number of pins, but there are changes in the supporting circuitry and basic filament voltage. According to designer Ken Stevens, the 6922 is a better input tube than the 12AU7, and allows the SL-1's noise level to be reduced from -92dB to -112dB (unweighted). Apart from the noise figure, the electrical specifications have stayed the same: gain is within ±0.1dB of the Mk.II; there is no change in rise time, bandwidth, or slew rate. Still, Stevens says the sonic improvement is such that he seriously considered renaming the preamp the SL-2.

Assuming that the sonic improvements are as significant as claimed, the obvious question is, Why he hasn't used this tube before? In fact, the 6922 has been part of the SL-1's tube complement, but not in the line-input stage. The SL-1's design calls for the tube in this spot to have a plate voltage of 175V, and published 6922 specifications state that the maximum allowable plate voltage is 130V. The tube that Stevens now uses as the line-input tube is made by Sovtek in Russia, and has the European designation of E88CC. Although labeled 6922, it has—as Stevens discovered when reading the instruction sheet packed with the tubes—a maximum plate-voltage spec of 220V, well within the SL-1's requirements. To accommodate the new tube, the gain was padded down. There are no other circuit changes.

So how does the latest CAT sound? It sounds great! Not having used it with super-sensitive speakers, I hadn't found the previous Mk.II version to be particularly noisy. But the new one is definitely quieter: with the volume set at what corresponds to a loud listening level, I have to have my ear within a few inches of the speaker to hear any noise at all. That's with the line stage. On phono, there's still a bit of hiss—less than before—but it's masked when playing music.

Beyond the improvement in S/N ratio, although possibly related to it, the new CAT is more lively yet better behaved than in its previous incarnation. There's a definite improvement in dynamics, and the top is more open and extended, without any vestige of hardness or extra brightness. The resolution of fine detail—which was already one of the CAT's strengths—is better than ever.

As part of the update, my unit was supplied with all new tubes, and, as typically happens with new tubes, there was initially a bit of top-end emphasis. But as the tubes grew accustomed to their new home, playing music on and off for a couple of weeks, the sound regained its tonal neutrality.

CAT SL-1 Signature owners who had shelled out from $695 to $995 upgrading to the Mk.II may not like the idea of yet another upgrade, this one priced at $575, but I'd say the sonic improvement is definitely worth it. The production schedule at Convergent Audio Technology is such that updates are done only in the summer, so you have a bit of time to save up for it.

For prospective CAT owners, the news is all good: Despite the substantial improvement in sound quality, the SL-1 Signature MK.III is priced at $5950—the same as the Mk.II.—Robert Deutsch

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